1. Technical Field
The present invention relates to an abnormality judging apparatus which judges whether or not abnormality has occurred at a rotation angle detecting apparatus based on a sine wave signal and a cosine wave signal generated according to the rotation angle of a rotor detected by the rotation angle detecting apparatus.
2. Description of Related Art
In conventional electric power steering (EPS) systems with brushless motors, the rotation angle of the brushless motor is detected by using a resolver, and then the amount of electric current through the brushless motor is controlled based on the detected rotation angle. FIG. 1 is a block diagram of the configuration of a resolver and FIG. 2 is an explanatory drawing of the concept of conventional threshold values for use in abnormality detection.
In FIG. 1, reference numeral 11 denotes a rotor, i.e., a resolver rotor. The resolver rotor 11 has a coil 12 and is attached so as to rotate coaxially with the motor shaft of a brushless motor (not shown). An exciting coil 13, a sine coil 14, and a cosine coil 15 are placed adjacent to the resolver rotor 11. The exciting coil 13 is connected to an AC power supply 16. The sine coil 14 and the cosine coil 15 are connected to a signal processing unit 19 for processing output signals via interfaces 17 and 18 respectively.
When receiving an AC voltage from the AC power supply 16, the exciting coil 13 generates an excitation signal. The sine coil 14 and the cosine coil 15 each output a signal in response to the excitation signal. The outputted signals are transmitted to the signal processing unit 19 as sine wave signal and cosine wave signal, which are same in phase as the excitation coil 13, based on the rotation of the resolver rotor 11. The signal processing unit 19 generates a rotation angle signal, which indicates the rotation angle of the resolver rotor 11, i.e., the rotation angle of the motor shaft of the brushless motor, based on the sine wave signal and cosine wave signal, and outputs the rotation angle signal.
When the rotation angle signal cannot be outputted due to the occurrence of some abnormality such as shorting at the coil or the rotor in the resolver, the control and so on of the brushless motor cannot be normally exercised. Therefore there is a need to reliably detect the abnormality of the resolver.
In view of this, an apparatus has been described (in, for example, Japanese Patent Application Laid-Open No. 2002-104212) in which, for example, a general threshold value given by the relation cos2θ+sin2θ=1 is preset, the difference between the threshold value and a detected value obtained by the sum (cos2θ+sin2θ) of each square of a sine wave signal and a cosine wave signal outputted from a resolver is obtained, and it is judged that abnormality has occurred at the resolver when the difference is beyond a predetermined range.